For example, in a bone conduction speaker 31 provided in a cell phone using a conventional type of bone conduction speaker, generally speaking a cushioning material 34 is interposed between the bone conduction speaker 32 and a housing 32 within a concave portion 33 of the housing 32 so as to keep the bone conduction speaker in effective isolation, wherein the concave portion 33 is provided in a suitable area of the housing 32 and has its depth extended in a direction of the thickness of the housing 32. The cushioning material 34 is provided in either a bottom surface of the concave portion 33 only (FIG. 10) or an area extending from such bottom surface to a side surface of the concave portion 33.
In the case of the cell phone using the above-mentioned conventional type of bone conduction speaker, such cell phone is convenient since the cell phone is capable of clearly catching a received sound even in a high-noise environment. On the other hand, the cell phone of this type is disadvantageous in that the cell phone of this type is larger in thickness than a cell phone using a conventional sound-pressure speaker since the cell phone of this type is provided with the concave portion for receiving therein the bone conduction speaker as described above and the cushioning material must be disposed on the bottom surface of the concave portion. The above disadvantage is remarkable in a foldable type of cell phone since an increase in thickness of the cell phone causes discomfort to its user.
In the above-mentioned construction, in order to reduce the cell phone in thickness, the cushioning material used in the cell phone is small in thickness. However, such thin-thickness cushioning material is poor in a cushioning effect. This makes it impossible to keep the bone conduction speaker in effective isolation with respect to the housing. As a result, echo back increases in volume. Due to this, in order to prevent such large echo back from being produced, it is impossible for the cell phone using the conventional bone conduction speaker to issue an output large in volume. This is a problem inherent in the cell phone using the conventional bone conduction speaker.
The same as described in the above is true also in the case of a bone conduction microphone. More specifically, the bone conduction microphone has substantially the same construction as that of the bone conduction speaker, provided that some differences are recognized between the bone conduction speaker and the bone conduction microphone in function in that the bone conduction microphone picks up a bone conduction sound through its diaphragm or vibration plate, whereas an output of the bone conduction speaker is issued from its diaphragm or vibration plate.    Patent document 1: Japanese Patent Application Laid-Open No. 2003-348208; and    Patent document 2: Japanese Patent Application No. 2002-352000